WordPress 4.0 RC1  has been released and the final version is targeted to be shipped next week.
Developers, please test your plugins and themes against WordPress 4.0 and update your plugin’s Tested up to version in the readme to 4.0 before next week. If you find compatibility problems, please be sure to post any issues to the support forums so we can figure those out before the final release. You also may want to give your plugin an icon, which we launched last week and will appear in the dashboard along with banners.
Download: WordPress 4.0 RC 1